1. Field of the Invention
This invention relates to a sensing device utilizing magneto electric transducers for measuring electric current or electric power. The sensing device according to this invention has a particular feature of compact size and high precision and can be used for both direct current and alternating current.
2. Description of the Related Art
There are many types of instruments used to measure electric current. Generally speaking, electric current is detected or measured by an instrument using a moving coil or a rotatable iron piece that indicate the measured data. These elements are placed between permanent magnet pole pieces or current carrying fixed coils. Such instruments have a comparatively large size, and is subject to insulation problems, especially when used in a high voltage circuit. Another type of instrument uses a current transformer. This instrument can measure large currents without the problem of insulation. This other instrument is, however, large and cannot easily measure a current having high frequency components.
Recently, magneto electric transducers, such as a Hall-effect element, and magneto-resistive elements have been used to measure electric current. These elements measure electric current with-out being directly inserted into a load circuit. Instead, these elements detect a magnetic field generated about a conductor through which a load current is flowing. These elements are therefore isolated from the load circuit, and have many applications in detecting and measuring.
A Hall-effect element has an electric current flowing through a material (usually a semiconductor material) located in a magnetic field that is perpendicular to the direction of the current. The magnetic field produces a voltage perpendicular to both the current and the magnetic field. When the magnetic field applied to the Hall-effect element is produced by a load current to be measured, the voltage output by the Hall-effect element is dependent on the load current and the characteristics of Hall-effect element itself.
A magneto-resistive element has a current flowing through a ferromagnetic material. The resistance of the material changes when the material is subjected to a magnetic field. When the magnetic field is generated by a load current, the current through the magneto-resistive element changes depending on the load current.
FIG. 1 illustrates an example current measuring instrument using a magneto-resistive element. This instrument is disclosed in Japanese Tokukaisho 59-79860 by Matsumoto. FIG. 1, an electric current to be measured flows through a wire 1. FIG. 1 illustrates the instrument with the electric current flowing in a direction into the paper. The magneto-resistive layer 2 comprises a ferromagnetic material such as Fe-Ni alloy. The layer 2 is formed in a strip shape on an insulating substrate 5, and two terminals 3, 4 are formed on both ends of the magneto-resistive layer 2. A magnetic core 6 is positioned about the wire 1 so that a magnetic field generated around the wire 1 can easily form a magnetic loop 7 which flows through the magnetic core 6 and the magneto-resistive layer 2. The resistance between the terminals 3 and 4 can be measured by applying an constant current to the layer 2 and measuring the voltage drop across the terminals 3 and 4. The measured voltage and applied current define the resistance. Resistance changes can be detected when a magnetic field is generated by a load current flowing through the wire 1 by, for example, detecting changes in the current. The magnitude of the load current can therefore be determined from the amount of resistance change.
A current measuring instrument such as illustrated in FIG. 1 requires a relatively large magnetic core 6. In addition, the ambient (e.g., the earth's magnetic field), causes errors in current measurements of this instrument. This is because the ambient magnetic field causes unwanted changes in the resistance of a layer 2.
In a similar way, electric current can be measured using a Hall-effect element which is shown in FIG. 2. A constant voltage source V connected across terminals Ta causes a small current i to flow through a Hall-effect element 8. A load current I flows through terminals Ta' and generates a magnetic field B vertical to the Hall-effect element 8. A measurement device 9 detects the Hall voltage induced across the Hall-effect element; the Hall voltage being perpendicular to both the current i and the magnetic field B.
The Hall-effect element shown in FIG. 2 can measure electric power by connecting terminals Ta to a power source which simultaneously supplies a load current through terminals Ta'. The induced Hall voltage (which is proportional to the electric power), detected by 9, can be measured. The detected Hall voltage is proportional to power because the current i through Hall-effect element 8 is proportional to the voltage V of power source and the applied magnetic field B is proportional to the load current I. When a DC power source is used, a DC power output is directly obtained. On the other hand, when an AC power source is used, the Hall voltage varies with a frequency that is double the frequency of the AC power source. This is because both the magnetic field direction and the direction of the current flow in the Hall-effect element reverse each half cycle. When the Hall voltage is averaged, it is proportional to electric power.
When a Hall-effect element is used in a measuring instrument, it is necessary to shield the element from the outside or ambient magnetic fields, especially from the terrestrial or earth's magnetic field. As a result, Hall-effect instruments are large and bulky.